1. Field of the Invention
The invention relates to a steam turbine which is operated in the part-load range with nozzle group control.
2. Discussion of Background
In steam turbine construction, control wheels having separately opening nozzle groups are used for power control because the efficiency which can be attained thereby is better, when considered over the significant power range, than in the case of other systems. The control wheel itself has the effect that work is drawn from the steam in such a way that power control in itself acts optimally.
In order to achieve full admission in the following stages, an equalization space is provided, which makes possible the transition from partial admission to full admission.
However, this is accomplished only incompletely, and therefore a nonuniform flow onto the reaction stages is produced. This flow inhomogeneity has the effect that the blades of the stages following the control wheel are subjected to harmful vibrations, which sometimes can lead to damage.
In order to avoid the nonuniform oncoming flow mentioned, various proposals have become known:
(a) Enlargement of the wheel space.
Whilst in the case of steam turbines of the smallest power class this is feasible within close limits, because so much space can be gained between the plane of discharge of the control wheel subjected to partial admission and the plane of entry of the first stage subjected to full admission that the flow downstream of the control wheel up to entry into the following part of the turbine can be made substantially uniform, in the case of steam turbines of greater power, which are characterized by large quantities of steam and large rotor diameters, it is constructionally not possible to design an intended control wheel much larger with respect to the following part subjected to full admission. Consequently, the space between the control wheel subjected to partial admission and the following part of the turbine subjected to full admission, for making the flow uniform over the entire circumference of the flow duct, remains small, as a result of which flow inhomogeneities are retained.
(b) Enlargement of the difference in diameter between control wheel and following parts of the turbine.
In fact, the argumentation presented under (a) applies here as well. By enlarging the diameter of the control wheel with respect to the diameter of the following stages, so much space is gained only in the case of steam turbines of the smaller power class between the plane of discharge of the control wheel subjected to partial admission and the plane of entry of the first stage subjected to full admission that the greatest flow inhomogeneities can be intercepted.
(c) Installation of restricting elements such as baffle or eddy space.
These precautions are, on the one hand, too complicated and, on the other hand, have adverse effects on the efficiency of the machine.
(d) Introduction of a swirl space downstream of the first reaction guide row.
This measure lengthens the machine unnecessarily, with all the constructional and economic disadvantages this entails.